During their time as correspondents in China, WuDunn and Kristof learned of the phenomenon of an estimated 30 million "missing" baby girls in the nation.WuDunn says part of the gap could be attributed to infanticide by families who were determined to have a male child under China's one-child policy and in part to the development of the sonogram. That medical device can be used to determine the gender of a child before birth, prompting some parents to obtain abortions."One peasant in the southern part of China once told us, 'The sonogram's great, we don't need to have baby girls any more.'